BILLINGS, Mont. – As efforts falter to save North America’s largest freshwater fish — a toothless beast left over from the days of dinosaurs — officials hope to stave off extinction by sending more water hurtling down a river so the fish can spawn in the wild.

The U.S. Fish and Wildlife Service on Thursday declared that attempts over the past two years to save the endangered Kootenai Riverwhite sturgeon had failed.

The prehistoric sturgeon, characterized by its large head and armor-like scales, can reach 19 feet long and top 1,000 pounds.

An isolated population of the species lives along a stretch of the Kootenai that passes through Montana, northern Idaho and southern British Columbia. Fewer than 500 of the bottom-feeding behemoths survive — and it’s been 35 years since they successfully spawned.

The problem is Libby Dam, a hydroelectric facility in Montana run by the Army Corps of Engineers that serves power markets in the Pacific Northwest. When the dam went up in 1974, it stopped periodic flooding ofBonners Ferry, Idaho — but also high water flows that triggered the sturgeon to move upriver and spawn.

After years of litigation, the federal government agreed to alter how it runs the dam and more closely mimic historical water flows. That hasn’t worked, and fisheries officials and the Corps now say they plan to spill more water over the dam next spring.

It could be one of the last chances to stave off disaster for the massive fish: Biologists say it could be on track for extinction within the next decade unless a fix is found.

Even with the increased spillover from the dam, the Kootenai River would rise to less than half its historical levels.

“We’re still kind of tinkering around at the bottom end of what historically used to occur,” said Jason Flory, a biologist with the U.S. Fish and Wildlife Service. “But the spring flows that were pre-Libby Dam were what flooded Bonners Ferry. You just don’t do that, you don’t flood towns.”

Before the dam, there were an estimated 10,000 Kootenai sturgeon.

White sturgeon also are found in the Columbia River, but the Kootenai population is genetically distinct after being isolated since the last Ice Age. There are 24 species of sturgeon worldwide; most are threatened with extinction.

The plan to save the fish in the Kootenai came out of a 2008 settlement with an environmentalists who sued the federal government for failing to take action.

Noah Greenwald, endangered species director for the group behind the lawsuit, the Center for Biological Diversity, said Thursday he was “cautiously optimistic” the new measures would work.

“There’s no way of knowing if it will be successful or not,” he said. “We certainly hope so, because the sturgeon is running out of time.”

Since the mid-1990s, Idaho’s Kootenai Tribe of Indians has stocked the river with thousands of hatchery-raised sturgeon in an attempt to fill the vacuum created as older fish die off.

To satisfy the requirements of the Endangered Species Act, the fish would have to reproduce naturally before the species is considered recovered.

It’s not known if the ploy will work. And because it takes 20 or 30 years for white sturgeon to mature and reproduce, the older fish will likely be gone before researchers find out.